Easier plugin debugging by noting source filename of internal exceptions

[Zac-HD]

I patched this in while trying to track down where in a rather large codebase python-trio/flake8-async#322 was triggered, and then thought that it was useful enough and had so little downside that I should open this PR 🙂

[asottile]

for debugging I'd recommend setting a breakpoint instead!

[Zac-HD]

What's the downside of this?

Of course I can pull out a debugger out if I need to, and get the raw invocation instead of going via whatever tooling, but adding a note means I don't even need to because the file responsible is printed right there when I first discover something is wrong.

[asottile]

What's the downside of this?

• version specific code
• untested
• maybe (?) breaks the stacktrace due to the AttributeError (idk I'd have to check)
• the exception output is potentially confusion with the existing messages